pubs.acs.org/Langmuir © 2009 American Chemical Society
Hierarchical Assembly of an Achiral π-Conjugated Molecule into a Chiral Nanotube through the Air/Water Interface Pingping Yao, Haifeng Wang, Penglei Chen,* Xiaowei Zhan,* Xun Kuang, Daoben Zhu, and Minghua Liu* Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, 100190, China Received April 22, 2009. Revised Manuscript Received May 13, 2009 An achiral π-conjugated fluorinated fused pyrazine derivative has been spread at the air/water interface, and its assembling property is investigated. It has been found that the compound, although without any long alkyl chain, could be spread as a floating film on water surface, the surface pressure of which can be compressed up to ca. 70 mN/m. An inflection point has been observed in the isotherm of the floating film on water surface. The atomic force microscope (AFM), scanning electron microscope (SEM) as well as the transmission electron microscope (TEM) observations revealed that the floating film first formed a multilayer structure and then was compressed into nanotubes after the inflection region as a result of the rolling of the ultrathin film. Interestingly, the rolled nanotubes show circular dichroism although the molecule itself is an achiral species, suggesting the chiral nanotube is predominantly produced on the water surface. The investigation provides an effective way to fabricate supramolecular-based organic chiral nanotubes through an interfacial supramolecular assembly process.
Introduction Hierarchical self-assembly has recently been recognized as one of the most important “bottom-up” strategies for the fabrication of diverse nanostructures.1 Among various nanostructures, there is an increasing interest in nanotubes because of their elegant structures and potential applications in various fields.2 Bedsides the well-known carbon nanotubes,3 the fabrication of supramolecular-based organic nanotubes through a supramolecular self*Corresponding author. Fax: (+) 86-10-62569564. Tel: (+) 86-1082612655. E-mail:
[email protected] (P.C.);
[email protected] (X.Z.);
[email protected] (M.L.). (1) For reviews, see: (a) Ryu, J.-H.; Hong, D.-J.; Lee, M. Chem. Commun. 2008, 1043–1054. (b) Ariga, K.; Hill, J. P.; Lee, M. V.; Vinu, A.; Charvet, R.; Acharya, S. Sci. Technol. Adv. Mater. 2008, 9, 014109. (c) Koblenz, T. S.; Wassenaar, J.; Reek, J. N. H. Chem. Soc. Rev. 2008, 37, 247–262 and the references therein. (2) For reviews, see: (a) Xiong, Y.; Mayers, B. T.; Xia, Y. Chem. Commun. 2005, 5013–5022. (b) Tenne, R. Nat. Nanotechnol. 2006, 1, 103–111. (c) Golberg, D.; Bando, Y.; Tang, C.; Zhi, C. Adv. Mater. 2007, 19, 2413–2432. (d) Bae, C.; Yoo, H.; Kim, S.; Lee, K.; Kim, J.; Sung, M. M.; Shin, H. Chem. Mater. 2008, 20, 756– 767. (e) Li, X. J. Phys. D: Appl. Phys. 2008, 41, 193001. (f) Steinhart, M.; Wehrspohn, R. B.; G€osele, U.; Wendorff, J. H. Angew. Chem., Int. Ed. 2004, 43, 1334–1344. (g) Remskar, M. Adv. Mater. 2004, 16, 1497–1504. (h) Demontis, F. PLoS Biol. 2004, 2, 0896–0897 and the references therein. (3) For reviews, see: (a) Hersam, M. C. Nat. Nanotechnol. 2008, 3, 387–394. (b) Zhang, S.; Kumar, S. Small 2008, 4, 1270–1283. (c) Allen, B. L.; Kichambare, P. D.; Star, A. Adv. Mater. 2007, 19, 1439–1451. (d) Kauffman, D. R.; Star, A. Chem. Soc. Rev. 2008, 37, 1197–1206. (e) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006, 106, 1105–1136. (f) Lu, F.; Gu, L.; Meziani, M. J.; Wang, X.; Luo, P. G.; Veca, L. M.; Cao, L.; Sun, Y.-P. Adv. Mater. 2009, 21, 139–152 and the references therein. (4) For reviews, see: (a) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. Rev. 2005, 105, 1401–1443. (b) Zhou, Y.; Shimizu, T. Chem. Mater. 2008, 20, 625–633. (c) Brizard, A.; Oda, R.; Huc, I. Top. Curr. Chem. 2005, 256, 167–218. (d) Elemans, J. A. A. W.; Rowan, A. E.; Nolte, R. J. M. J. Mater. Chem. 2003, 13, 2661–2670. (e) He, Q.; Cui, Y.; Ai, S.; Tian, Y.; Li, J. Curr. Opin. Colloid Interface Sci. 2009, 14, 115–125. (f) Yamamoto, T.; Fukushima, T.; Aida, T. Adv. Polym. Sci. 2008, 220, 1–27 and the references therein. (5) For reviews, see: (a) Steinhart, M. Adv. Polym. Sci. 2008, 220, 123–187. (b) Liu, G. Adv. Polym. Sci. 2008, 220, 29–64. (c) Cho, S. I.; Lee, S. B. Acc. Chem. Res. 2008, 41, 699–707. (d) Aleshin, A. N. Adv. Mater. 2006, 18, 17–27. (e) Hamley, I. W. Soft Matter 2005, 1, 36–43 and the references therein. (6) For reviews, see: (a) Scanlon, S.; Aggeli, A. Nano Today 2008, 3, 22–30. (b) Gao, X.; Matsui, H. Adv. Mater. 2005, 17, 2037–2050. (c) Numata, M.; Shinkai, S. Adv. Polym. Sci. 2008, 220, 65–121 and the references therein.
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assembly is an important part of the research on nanotubes.4-6 In particular, organic nanotubes composed of π-electron molecules provide a large design flexibility in functionalization, while nanostructures in nature with sophisticated hierarchy, such as protein tubes, provide excellent examples of integrated functions and structures of the organic or biological nanotubes.7 Thus, the rational design of molecular structures and optimized control of self-assembly conditions in forming the organic nanotubes have drawn considerable attention. Various amphiphilic molecules, biorelated molecules, and polymer molecules have been designed, and their formation of nanotubes has been widely investigated.4-6 In comparison with the lager amount of work on the formation of nanotubes in a three-dimensional space through a hierarchical assembly in solutions, there are only a few reports on the formation of nanotubes in a two-dimensionally confined interface, while the latter could possibly provide a clearer understanding of the formation mechanism of the nanotubes.8 In this paper, we report some new insight into the fabrication of nanotubes through a two-dimensional air/water interface by the Langmuir-Blodgett technique. The Langmuir as well as Langmuir-Blodgett (LB) techniques have been used to fabricate the uniform films of amphiphilic molecules for a long time.9 However, recent investigations show that this technique is also an effective strategy for producing various nanostructures. So far, diverse nanostructures such as patterned nanoarrays, nanostripes, nanofibers, and nanoparticles (7) (a) W€urthner, F.; Chen, Z.; Hoeben, F. J. M.; Osswald, P.; You, C.-C.; Jonkheijm, P.; Van Herrikhuyzen, J.; Schenning, A. P. H. J.; Van der Schoot, P. P. A. M.; Meijer, E. W.; Beckers, E. H. A.; Meskers, S. C. J.; Janssen, R. A. J. J. Am. Chem. Soc. 2004, 126, 10611–10618. (b) Messmore, B. W.; Sukerkar, P. A.; Stupp, S. I. J. Am. Chem. Soc. 2004, 126, 7992–7993. (c) Bae, J.; Choi, J.-H.; Yoo, Y.-S.; Oh, N.-K.; Kim, B.-S.; Lee, M. J. Am. Chem. Soc. 2005, 127, 9668–9669. (d) Schuyler, S. C.; Pellman, D. Cell 2001, 105, 421–424. (8) (a) Guo, L.; Wu, Z.; Liang, Y. Chem. Commun. 2004, 1664–1665. (b) Shankar, B. V.; Patnaik, A. Langmuir 2006, 22, 4758–4765. (9) Ulman, A. An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic Press: Boston, 1991.
Published on Web 05/21/2009
DOI: 10.1021/la901435s
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Figure 1. Surface pressure (π)-area (A) isotherm of FFP on a water surface at 20 °C. Inset: a three-dimensional model of an FFP molecule calculated by the energy optimizations from force-field calculations MM2.
have been fabricated by the LB technique.10 A fewer papers have also reported the formation of nanotubes in LB assemblies.8 It is suggested that the nanotubes in these Langmuir films are formed through the rolling of the nanosheet, which can be regulated through film compression at the air/water interface. When considering the rolling of the nanosheet, both theoretical models and experimental results suggested that, in forming high-curvature nanotubes, a chiral packing is usually needed.11 It is no problem for chiral molecules to form such chiral packing, just as in the case of our previous reported chiral bolaamphiphile.12 However, it remains a question how achiral species form such chiral packing. More importantly, when the achiral molecules are rolled into nanotubes, are these nanotubes chiral or achiral? In this communication, we have investigated the hierarchical assembly of a π-conjugated fluorinated fused pyrazine (FFP, inset in Figure 1),13 which has no long alkyl chains on the molecular skeleton. We have disclosed that, through a compression at a two-dimensional air/water interface, the molecules can be hierarchically assembled into nanotubes. We have further revealed for the first time that, through such compression, the nanotubes with one-handedness, which are composed of achiral molecules, can be predominantly produced through the LB method.
Results and Discussion Figure 1 shows the surface pressure-molecular area (π-A) isotherm of the spreading film of FFP on a water surface. The isotherm shows the onset of surface pressure at a molecular area of ca. 1.2 nm2/molecule. Upon compression, a rapid rise of surface pressure is seen until about 0.8 nm2/molecule, after which an inflection point is observed. After the transition point, the film shows a second rapid rising of the surface pressure, which can be compressed up to ca. 70 mN/m. On the basis of the molecular modeling (inset in Figure 1), the molecule can be regarded as a cuboid box, and the dimensions are estimated to be 1.9 nm 1.1 nm 0.34 nm. Thus, it can be suggested that, instead of lying with their largest face on the water surface, the molecules aligned with their long side inclined to the water surface at the (10) For reviews, see: (a) Chen, X.; Lenhert, S.; Hirtz, M.; Lu, N.; Fuchs, H.; Chi, L. Acc. Chem. Res. 2007, 40, 393–401. (b) Tao, A. R.; Huang, J.; Yang, P. Acc. Chem. Res. 2008, 41, 1662–1673 and the references therein. (11) (a) Papadimitrakopoulos, F.; Ju, S.-Y. Nature 2007, 450, 486–487. (b) Wild€oer, J. W. G.; Venema, L. C.; Rinzler, A. G.; Smalley, R. E.; Dekker, C. Nature 1998, 391, 59–62. (c) Barros, E. B.; Jorio, A.; Samsonidze, G. G.; Capaz, R. B.; Filho, A. G. S.; Filho, J. M.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rep. 2006, 431, 261–302. (12) Gao, P.; Liu, M. Langmuir 2006, 22, 6727–6729. (13) Wang, H.; Wen, Y.; Yang, X.; Wang, Y.; Zhou, W.; Zhang, S.; Zhan, X.; Liu, Y.; Shuai, Z.; Zhu, D. ACS Appl. Mater. Interfaces, ASAP paper. DOI: 10.1021/am900093p.
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initial spreading, which might be due to the hydrophobic property of the CF3 group.14 Upon compression, the molecules could possibly stack into multilayer films as a result of the lack of an alkyl chain. In order to further investigate the molecular stacking in different stages of the compression isotherm, the floating film was transferred onto solid supports by a horizontal lifting method at various surface pressures, and the atomic force microscope (AFM), scanning electron microscope (SEM), and transmission electron microscope (TEM) images of the transferred films were measured, as shown in Figure 2. Domains are observed even for the films deposited at 0 mN/m (Figure 2A). The height of these domains is ca. 1 nm, which is close to the length of the short side of the molecule calculated by the energy optimizations from forcefield calculations MM2. This suggests that the molecules stand on the water surface with the long axis nearly parallel to the water surface, when the surface pressure is 0 mN/m. The AFM image of the film deposited at 5 mN/m exhibits large island domains accompanied by a few rod-like nanostructures. For the film deposited at 30 mN/m, nanorod-like morphologies are mainly observed. In order to further disclose the nature of these rod-like nanostructures formed at higher surface pressures, we further measured the SEM and TEM images of the transferred films at 30 mN/m, as shown in Figure 2D,E. Nanotubes are clearly seen in the SEM pictures. TEM observation has confirmed such nanotube structure and further revealed that the nanotube is a multiwalled one. The average thickness of each wall is estimated to be ca. 3.1 nm. On the other hand, when we measured the TEM for the film deposited at a lower surface pressure, to say, 5 mN/m, coexistence of the nanosheet and nanotubes is observed (Figure 2F). In addition, the nanotube appears at the edge of the nanosheet. This clearly indicates that the nanotube is formed by the rolling of the nanosheet. Since the molecule has a π-conjugated system, the film was transferred onto quartz plates to further characterize their optical properties. Figure 3 shows the UV-vis spectra of the transferred film in comparison with the compound in chloroform solution. In solution, two strong peaks appear at 275 and 310 nm, and a relatively weak broad peak appears at 406 nm accompanied by a shoulder peak at 425 nm. When the film is transferred onto a solid substrate, a slight shift of the absorption bands is observed, where the band at 275 nm blue shifts to 266 nm, while the two bands appearing at longer wavelengths shift to 410 and 432 nm, respectively. The former band can be ascribed to the transition due to the short axis, while the latter absorption band could be due to the transition moment along the long axis. The data indicates that the molecules are stacked in a head-to-tail way in the long axis, while they are stacked in the short axis in a face-toface way. In addition, when the surface pressure is changed from 5 to 30 mN/m, no UV-vis spectral changes could be observed (Figure 3A, bottom panel). This indicates that, in the film deposited at either 5 or 30 mN/m, there is no difference in the π-π stacking manner of the molecules in the film. Therefore, the compression of the floating film on water surface mainly changes the morphology of the films. Interestingly, when measuring the circular dichroism (CD) spectra of the film,15 a strong CD signal is detected for the film deposited at 30 mN/m, although we could not detect any CD (14) Kissa, E. Fluorinated Surfactants: Synthesis, Properties, Applications (Surfactant Science Series); Marcel Dekker: New York, 1994; Vol. 50. (15) (a) Spitz, C.; D€ahne, S.; Ouart, A.; Abraham, H. W. J. Phys. Chem. B 2000, 104, 8664–8669. (b) Yuan, J.; Liu, M. J. Am. Chem. Soc. 2003, 125, 5051–5056. (c) Huang, X.; Li, C.; Jiang, S.; Wang, X.; Zhang, B.; Liu, M. J. Am. Chem. Soc. 2004, 126, 1322–1323.
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Letter
Figure 2. AFM images of the films of FFP deposited at 0 (A), 5 (B), and 30 mN/m (C). Image size: 2 μm 2 μm. (D) SEM image of the film deposited at 30 mN/m. (E,F) TEM images of the film deposited at 30 mN/m (E) and 5 mN/m (F).
Figure 3. UV-vis spectra (A, bottom) of FFP in chloroform solution (black) and in films deposited at 5 (red) and 30 (green) mN/m; CD spectra (A, top) of FFP in chloroform solution (black) and in films deposited at 5 (red) and 30 (green and blue) mN/m. The green and blue CD curves are measured from the films deposited in different batches, while the curves with the same color (green or blue) refer to the CD spectra measured at different positions in the same films. (B) the CD and LD spectra of the films deposited at 30 mN/m, which were unified as the same unit (Δ OD) and made comparable by using a semiempirical equation.16
signal from the solution of the compound and from the films deposited at 5 mN/m. In one of the CD spectra, positive Cotton effect (CE) is observed at 425, 405, 316, and a negative one is seen at 254 nm, which are close to their UV-vis absorption bands. This means that our nanotubes are chiral. Since the CD measurement of the film could contain some artifacts, we have carefully measured the CD spectra of the deposited films to avoid such artifacts in the following manner: First, we put the film perpendicular to the light path and rotated the film within the film plane continuously to eliminate the possible artifacts caused by the anisotropic arrangements of the molecules or supramolecular structures. Second, we measured the LD spectra of the film to estimate the contribution from the LD effect, as shown in Figure 3B. On the basis of these measurements, it is clear that the contamination of CD by the linear dichroism (LD) artifact is limited to ca. 8%.16 These spectral features support the chirality of the formed nanotube. On the other hand, when measuring the CD spectra of the films fabricated in different batches, opposite CD signals could be detected, as shown in Figure 3A. For a film showing positive (or negative) CD signals, CE with different (16) (a) Tsuda, A.; Alam, Md. A.; Harada, T.; Yamaguchi, T.; Ishii, N.; Aida, T. Angew. Chem., Int. Ed. 2007, 46, 8198–8202. (b) Ohira, A.; Okoshi, K.; Fujiki, M.; Kunitake, M.; Naito, M.; Hagihara, T. Adv. Mater. 2004, 16, 1645–1650. (c) Gillgren, H.; Stenstam, A.; Ardhammar, M.; Norden, B.; Sparr, E.; Ulvenlund, S. Langmuir 2002, 18, 462–469.
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intensity could also be detected in the different regions of the same film. Occasionally, opposite CD signals could be detected from different regions of the film deposited in the same batch. This means that the chiral nanotubes were formed through a symmetry breaking during the assembly at the interface. Furthermore, the film has also been deposited at 68 mN/m, where the floating film is collapsed. Bundles of the nanotubes were observed from the SEM (Figure S5, A, Supporting Information). In addition, the film showed CD spectra similar to those of the films deposited at 30 mN/m. This indicated that both the chirality and the nanotube structures were basically kept when the film was further compressed to collapse. On the basis of these experimental results, a mechanism can be proposed for the interfacial formation of the chiral nanotubes, as shown in Scheme 1. When the compound is spread onto a water surface, it forms a monolayer at the air/water interface at very low surface pressure. Upon compression, the compound formed multilayer nanosheet films easily as a result of the lack of long alkyl chains. Owing to the exceptional hydrophobicity of the fluorinated compound, such nanosheets tend to be rolled up into nanotubes upon further compression. Since the rolling process is limited in a two-dimensional space, the rolling is sometime incomplete. Therefore, we could also observe the seminanotube in the TEM. DOI: 10.1021/la901435s
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compounds.8 Unfortunately, they did not characterize the possible chirality of the formed nanotube. Since the rolling of the nanosheet into the nanotube usually accompanies the asymmetry, their nanotube might be chiral also.
Conclusions
On the other hand, when the nanosheet rolled up into a nanotube, the direction of the curvature and the rolling of the nanosheet could produce chirality, as illustrated in Scheme 1. If chiral molecules were used, they tended to form one-handed nanotubes based on the chirality of the molecules. For achiral molecules, the curvature and rolling of the nanosheet occurred by chance, and, in most cases, an equal mole of the two oppositehanded nanotubes will be obtained. However, in a two-dimensionally confined space, the one-handedness of the nanotube was supposed to predominantly occur at the start of the rolling and be amplified upon compression. Thus, we detected the chirality of the rolled nanotubes. The initial predominant curving is formed by chance. Therefore, we detected the opposite CD signals in different batches. Previously, Liang et al. and Patnaik et al. have reported the formation of nanotubes for long alkyl chain
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In summary, we have disclosed that chiral supramolecular nanotubes could be fabricated through the interfacial scrolling of a floating film of a π-conjugated fluorinated compound. The described methodology might open up a promising way for the fabrication of chiral supramolecular nanotubes by using achiral units. Further work concerning a detailed mechanism is currently in progress in our lab. Acknowledgment. We thank the 973 Program (2007CB808005, 2006CB932100), NSFC (20873159, 20533050, 20773141, 50873107, 20721061) and the Chinese Academy of Sciences for financial support. Supporting Information Available: Experimental details; data for identifying the authenticity of the CD signals and for the estimation of the contamination of CD by LD artifact; the SEM image, UV-vis, and CD spectra of the film deposited at collapse pressure. This material is available free of charge via the Internet at http://pubs.acs.org.
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